Due to, among other things, their small size, high Q values, and very low insertion losses at microwave frequencies, particularly those above 1.5 Gigahertz (GHz), Bulk Acoustic Wave (BAW) filters have become the filter of choice for many modern wireless applications. In particular, BAW filters are the filter of choice for many 3rd Generation (3G) and 4th Generation (4G) wireless devices. For instance, virtually all Long Term Evolution (LTE) compatible mobile devices operating in LTE frequency bands above 1.9 GHz utilize BAW filters. For mobile devices, the low insertion loss of the BAW filter provides many advantages such as, e.g., improved battery life, compensation for higher losses associated with the need to support many frequency bands in a single mobile device, etc.
One example of a conventional BAW resonator 10 is illustrated in FIG. 1A. In this example, the BAW resonator 10 is, in particular, a Solidly Mounted Resonator (SMR) type BAW resonator 10. As illustrated, the BAW resonator 10 includes a piezoelectric layer 12 (which is sometimes referred to as a piezoelectric plate) between a bottom electrode 14 and a top electrode 16. Since the BAW resonator 10 is a SMR type BAW resonator 10, the BAW resonator 10 also includes a reflector 18 (which is more specifically referred to as a Bragg reflector) that includes multiple layers 20-28 of alternating materials with varying refractive index. In this example, the BAW resonator 10 also includes a Border (BO) ring 30 on the top surface of the top electrode 16 around the periphery of the top electrode 16. Finally, the BAW resonator 10 includes a passivation layer 32.
In operation, acoustic waves in the piezoelectric layer 12 within an active region 34 of the BAW resonator 10 are excited by an electrical signal applied to the bottom and top electrodes 14 and 16. The active region 34 is the region of the BAW resonator 10 that is electrically driven. In other words, the active region 34 is the region of the BAW resonator 10 consisting of, in this example, the bottom electrode 14, the top electrode 16, the portion of the piezoelectric layer 12 between the bottom and top electrodes 14 and 16, and the portion of the reflector 18 below the bottom electrode 14. Conversely, an outer region 36 of the BAW resonator 10 is a region of the BAW resonator 10 that is not electrically driven (i.e., the area outside of the active region 34). The frequency at which resonance of the acoustic waves occurs is a function of the thickness of the piezoelectric layer 12 and the mass of the bottom and top electrodes 14 and 16. At high frequencies (e.g., greater than 1.5 GHz), the thickness of the piezoelectric layer 12 is only micrometers thick and, as such, the BAW resonator 10 is fabricated using thin-film techniques.
Ideally, in order to achieve a high Q value, the mechanical energy should be contained, or trapped, within the active region 34 of the BAW resonator 10. The reflector 18 operates to prevent acoustic waves from leaking longitudinally, or vertically, from the BAW resonator 10 into the substrate (not shown, but below the reflector 18). Notably, in a Film Bulk Acoustic Resonator (FBAR) type BAW resonator, an air cavity is used instead of the reflector 18, where the air cavity likewise prevents acoustic waves from escaping into the substrate.
While the reflector 18 (or air cavity for a FBAR type BAW resonator) confines mechanical energy within the active region 34 of the BAW resonator 10 in the longitudinal, or vertical, direction, a substantial amount of mechanical energy still leaks laterally from the active region 34 of the BAW resonator 10 into the outer region 36 of the BAW resonator 10 and then down into the substrate, as illustrated FIG. 1B. This lateral leakage of mechanical energy at the boundaries of the BAW resonator 10 degrades the Q of the BAW resonator 10. As such, there is a need for systems and methods for mitigating the loss of mechanical energy through lateral dispersion into the outer region 36 of the BAW resonator 10.